Cobalamin or vitamin B12 is a water-soluble vitamin which forms part of the vitamin B complex found in foods. The core molecule consists of a corrin ring of four pyrole units which surround the essential cobalt atom. Cobalamin is the only vitamin which cannot be synthesised by animals or plants and must be absorbed from food in the gut. It can however be stored in the liver. It is synthesised by micro-organisms, in particular by anaerobic bacteria and yeasts.
Cobalamin functions in vivo as a co-enzyme and cobalamin enzymes catalyse three types of reaction: (i) intra-molecular rearrangements, for example, the formation of succinyl CoA from L-methylmalonyl CoA; (ii) methylations, for example, the formation of methionine by methylation of homocysteine; and (iii) reduction of ribonucleotides to deoxyribonucleotides in some micro-organisms. In mammals, only two enzymic reactions, those specifically mentioned in (i) and (ii) above, are known to require cobalamin as a co-enzyme.
In the process of digestion, a salivary protein called haptocorrin, hereinafter referred to as HC (which is also referred to in the art as R-binder or transcobalamins I and III collectively), binds cobalamin in the upper gastrointestinal tract forming a complex which passes through the stomach. Pancreatic enzymes digest the cobalamin-haptocorrin (holo-HC) complex in the ileum, liberating cobalamin which is then bound to a protein called intrinsic factor, which is secreted by the gastric mucosa, to form a further complex. The cobalamin-intrinsic factor complex binds to a specific receptor in the lining of the terminal ileum, whereupon it is dissociated by a releasing factor and the cobalamin transported actively across the membrane of the ileum into the blood stream.
Cobalamin does not circulate in the body in a free form in an appreciable amount. Probably 99% or so of cobalamin is bound by haptocorrin, transcobalamin or albumin.
The protein believed to be responsible for transporting cobalamin to target tissues is transcobalamin II (hereinafter simply referred to as transcobalamin or TC), a critical trace protein without which cobalamin cannot cross cell membranes. Despite this important metabolic function, only about 6-25% of cobalamin in the serum is bound to TC and most is carried by HC. TC is a single chain polypeptide of 45 kDa found primarily in serum, seminal fluid and cerebro-spinal fluid. Cobalamin bound TC or holo-TC, attaches to specific receptors on cell membranes and once bound, the holo-TC complex is taken into cells by pinocytosis.
TC is synthesised by the liver, vascular endothelium, enterocytes, macrophages and fibroblasts and circulates predominantly as apo-TC, i.e. lacking bound cobalamin. It has a short half life of approximately 90 minutes.
Less than a quarter of the total plasma cobalamin is associated with TC. The rest is bound to HC or albumin as mentioned above. Since cobalamin must be absorbed from food, any conditions which result in impaired gastric function, for example, gastroenteritis or conditions resulting in gastric atrophy, or an inability to produce functional haptocorrin, intrinsic factor, releasing factor, TC or TC receptors, can result in impaired uptake of cobalamin and resultant deficiency.
Certain population sub-groups, for example the aged, pregnant women, patients with chronic or acute gastrointestinal disease, those suffering from certain autoimmune diseases, those with a family history of pernicious anaemia and AIDS sufferers, are particularly prone to cobalamin deficiency.
The clinical manifestations of cobalamin deficiency are varied and numerous but primarily involve anaemia, megaloblastic haematopoiesis and functional and structural disorders of the nervous system. Around 60% of individuals diagnosed as being deficient in cobalamin are anaemic, but in many neurological symptoms are the only clinical signs observed. Around 10% of patients exhibit psychiatric symptoms and around 40% exhibit both neurological and psychiatric symptoms.
Early diagnosis of cobalamin deficiency is crucial to ensure a good prognosis for patients, since some of the manifestations of cobalamin deficiency, particularly the neuropsychiatric effects, are irreversible if not detected and alleviated by cobalamin therapy quickly.
It is desirable therefore to accurately assess the cobalamin level of an individual in an expedient and efficient manner, with a view to establishing whether or not the individual may be suffering from cobalamin deficiency.
Measurement of total plasma cobalamin, i.e. cobalamin (and cobalamin like substances) bound to HC or TC, has been used in attempts to assess cobalamin deficiency. This technique results in a broad based concentration distribution within a population which is considered to be normal and hence produces a wide reference range. Within individuals however, the range of available cobalamin considered to be normal for that individual, is very narrow. It has been observed that although an individual's metabolically active cobalamin concentration has moved outside their own reference range, their total plasma cobalamin content remains within the range considered to be normal for the population. Under such circumstances, cobalamin deficiency can go undetected. Such an unreliable method is clearly undesirable and it is well recognised that such serum or plasma cobalamin measurements have low diagnostic sensitivity and specificity.
Microbial assays involving micro-organisms dependent upon cobalamin for growth, have been developed and used in measuring plasma cobalamin concentration, but in addition to the difficulty of estimating the appropriate reference range, these methods require extraction and conversion of the cobalamins which is very time consuming, troublesome and wholly unsuited for rapid laboratory screening.
Alternative methods for assessing cobalamin deficiency involve measuring the accumulation of metabolites in the plasma which require cobalamin for their conversion. Plasma methylmalonate and plasma homocysteine levels increase in cobalamin deficient individuals and make good candidate molecules for correlation with vitamin B12 deficiency. Methods based on homocysteine assessment have been shown, however, to be complicated, impractical and show poor specificity and sensitivity. Whilst methods based on methylmalonate measurement are accurate and reliable, they are cumbersome and require analysis by combined gas-chromatography/mass-spectrometry and are hence expensive and again unsuitable for routine clinical screening.
It has also been suggested that measurement of TC bound cobalamin as opposed to total plasma cobalamin may provide a reliable clinical indictor of the likelihood of cobalamin deficiency (Herbert et al. (1990) Am. J. Hematol. 34:132-139; Wickramasinghe and Fida (1993) J. Clin. Pathol. 46:537-539; U.S. Pat. No. 4,680,273). Such efforts to determine holo-TC concentration were mostly indirect, estimating holo-TC concentration as the difference between total plasma cobalamin and the cobalamin concentration of TC depleted plasma.
Such TC depletion may be accomplished by adsorption to ammonium sulphate (Carmel (1974) Am. J. Clin. Pathol. 62:367-372), microsilica (Herzlich & Hubert (1988) Lab. Invest. 58:332-337; Wickramasinghe & Fida (1993) J. Clin. Pathol. 46:537-539), microfine glass (Vu et al. (1993) Am. J. Hematol. 42:202-211) or immobilized anti-TC polyclonal antibodies (Lindemans et al. (1983) Clin. Chim. Acta 132:53-61). The concentration of cobalamin in total plasma and the depleted fraction is performed by methods well known in the art such as radio or enzyme immunoassay techniques. These methods are unsuitable for routine screening whether automated or not automated because they are complex and time consuming and because the low degree of specificity of the adsorptive materials used results in insufficient separation of holo-TC and holo-HC resulting in an overestimation of holo-TC. Lot-to-lot variation of the adsorptive material introduces further errors and most importantly, the subtraction of one large volume from another large volume results in unacceptable inaccuracies and unreliability.
Other attempts to assess TC have involved separating TC from other serum components, including HC, using its lipophilicity. Thus Kapel et al. (1988) Clin. Chim. Acta 172:297-310, Benhayoun et al. (1993) Acta Haematol. 89:195-199 and Toft et al. (1994) Scand. J. Clin. Lab. Invest. 54:62 disclose methods for separating TC from HC using heparin sepharose, silica gel or cellulose respectively. These methods however suffer from the same disadvantages as the indirect methods since they rely on the same adsorptive materials. Also, the low plasma concentration of holo-TC renders these methods unsuitable for combination with existing methods of cobalamin quantification. The normal range of holo-TC is 35-160 pM and values below 35 pM would generally be considered as indicative of cobalamin deficiency. The reported analytical sensitivity of most routine methods for plasma cobalamin is about 40 pM but in practice it is often much higher, typically around 90 pM. Hence, normal plasma levels of holo-TC are below or near the sensitivity limit of the routine methods for cobalamin quantification.
Possibly the most accurate method currently recognised for determining TC bound cobalamin involves adsorbing TC to silica and then assaying the bound fraction for cobalamin content using either an immunoassay as described for example by Kuemmerle et al. (1992) Clin. Chem. 38/10: 2073-2077, or a microbiological assay, the latter apparently producing the best results. This method requires an entire working day to perform only twenty assays. It is very expensive and impractical and poorly suited to routine clinical diagnostic laboratory investigations.
In WO 00/17659, an alternative method for determining holo-TC concentration in which, for example, a cell-free sample is contacted with an antibody specific for TC immobilized on magnetizable particles and with a non-overlapping, non-immobilized, radiolabelled antibody specific for holo-TC. The particles are separated out using a magnetic field and washed and their radioactivity is measured so providing a measurement of the holo-TC content of the sample. This method however requires calibration and is more suited for performance in a clinical laboratory than by a physician or physician's assistant at the point of care for the patient.
There thus still exists a need for an assay for holo-TC which is simple and operable at the point of care.